1. Technical Field
The present invention relates to compositions, particularly for coating paper, plastic film, or textiles, or wet end additives for papermaking, comprising aqueous solutions of cationic polyvinyl alcohol, more particularly the chloride quaternary salt of cationic polyvinyl alcohol.
2. Background Information
Cationic starch has long been used as a wet end additive for improving certain properties of paper sheets generated by the papermaking process. These properties include wet web strength, tensile strength, burst strength, and filler-loading capacity. The cationic charges on the starch molecules are believed to attract the starch to the anionically charged pulp fibers in a dilute aqueous dispersion. Not all of the starch is retained on the pulp fiber, though; excess starch is lost in the water expelled as a byproduct of the papermaking process. When the white water is discharged to a stream, lake, or other effluent discharge point, the high starch levels in the effluent increase biological oxygen demand (BOD). High BODs are known to disrupt the natural biological balance in the stream or other body of water to which the effluent is discharged.
On an equal film thickness or weight basis, polyvinyl alcohol has been reported to impart much greater wet web strength, tensile strength, etc. than starch; however, polyvinyl alcohol cannot be made cationic by the same inexpensive process as starch. An efficient process for preparing cationic polyvinyl alcohol has long been elusive.
A process is known for copolymerizing vinyl acetate and vinyl acetamide, then hydrolyzing the polymer to obtain what is apparently a copolymer of vinyl alcohol and vinyl amine. The product is neutralized with hydrogen chloride to make a hydrochloride salt, which is commercially available. Unfortunately, copolymers containing primary amine functional groups are often associated with paper yellowing problems. This is believed to be caused by these copolymers forming chromophores with glyoxal-containing paper additives during the papermaking process.
A process is also known for producing cationic polyvinyl alcohol by reacting a melamine-formaldehyde resin acid colloid with a solution of polyvinyl alcohol. Unfortunately, this process is tedious and time-consuming and its byproducts include formaldehyde, which is suspected to be a carcinogen.
Another process involves reacting a cationic polyacrylamide and blocked glyoxal resin with polyvinyl alcohol. This process is tedious, difficult to reproduce, and the product generally has a short shelf life.
The present invention includes compositions for coatings or wet end applications containing a graft copolymer of polyvinyl alcohol and a quaternized (meth)acrylic monomer. The chloride quaternary salt of cationic polyvinyl alcohol in particular has been found herein to be surprisingly and unexpectedly better than other quaternary salts of cationic polyvinyl alcohol, notably the dimethyl sulfate quaternary salt, for use in wet end applications and ink jet coatings. Furthermore, polyvinyl alcohol with certain molecular weight ranges and degrees of hydrolysis in the formulas herein convey particular advantages for the applications discussed herein.
The compositions of the present invention preferably include cationic polyvinyl alcohol of a controlled molecular weight range and degree of hydrolysis. The molecular weight and hydrolysis ranges are selected according to the anticipated end application. The formulas of the present invention offer great variety and consistency by controlling the degree of cationic charge possessed by the cationic polyvinyl alcohol. Formulas with a cationic polyvinyl alcohol with a greater or lesser cationic charge can be created according to the intended application. This is accomplished by selecting polyvinyl alcohol starting material with a certain molecular weight range and degree of hydrolysis, and by manipulating the amount and type of (meth)acrylic quaternary monomer and catalyst used in the process of manufacturing the cationic polyvinyl alcohol. Formulas containing cationic polyvinyl alcohol with a low cationic charge are preferred for wet end applications. Also, formulas containing polyvinyl alcohol with a higher molecular weight and a higher degree of hydrolysis are preferred as a starting point for wet end applications because they provide greater strength to the end product. Coating compositions containing cationic polyvinyl alcohol with higher amounts of cationic charge are preferred for use in coating paper, as well as plastic film and textile substrates used in ink jet printing. It is believed that this is because the anionic charges of the ink chromaphores are better immobilized by the cationic charges in the coating, giving a sharper image. Aqueous coating compositions containing lower molecular weight polyvinyl alcohol are preferred for coating, since higher solids can be achieved.
The present invention describes formulations and ratios for use in both wet end applications and ink jet coating applications. It is believed that the advantages of the formulations of the present invention are surprising and unexpected, and are not taught or suggested by prior art references.